The invention relates to a method for testing the operation of an electric heating element which is used for heating activated carbon of an activated carbon filter and/or air which is guided through the activated carbon for the regeneration thereof. In the case of one of the methods, the heating element itself has a PTC-characteristic (positive temperature coefficient characteristic). In the case of two further methods, an electric resistor which is electrically connected to the heating element has a PTC-characteristic. The resistor is a barretter and is designated hereinafter as a PTC-resistor.
When using a hybrid drive for a motor vehicle, which drive is characterized by an internal combustion engine and an electric motor, the time during which the internal combustion engine is in operation is reduced by the time during which the electric motor is operated. This situation is significant inter alia also for an activated carbon filter which is connected to a fuel tank serving to supply the internal combustion engine, as a regeneration of the activated carbon filter is limited to the times of operation of the internal combustion engine.
In general, the fuel tank is connected via a line to the ambient atmosphere, via which a pressure build-up and similarly a negative pressure are prevented in the head space of the tank and can be adjusted during refueling, during the removal of fuel or as a result of a temperature-induced evaporation of fuel. An activated carbon filter which is typically disposed in this line is intended to prevent hydrocarbons from passing into the environment in this way.
This type of activated carbon filter consists generally of a container which is arranged to receive activated carbon particles and is designed with connections to establish a connection to the fuel tank, the ambient atmosphere and the intake line of the internal combustion engine.
In order to reduce the amount of activated carbon to be used, the connection which is connected to the said intake line serves during a flushing phase to guide ambient air through the activated carbon filling of the filter, wherein by means of desorption the air absorbs adsorptively bonded hydrocarbons and introduces them into the combustion chamber of the engine, so that as a result the activated carbon is regenerated. However, this procedure necessitates operation of the internal combustion engine. However, since the amount of hydrocarbons, which issue out of the head space of the tank as a result of evaporation and which are to be absorbed in the activated carbon filling, is independent of the operation time of the vehicle, in the case of a hybrid drive the regeneration procedure must be brought to a conclusion in a relatively short time.
It is generally known that the conditions of a regeneration in terms of an acceleration can be improved by the thermal conditions of this conversion, in that the air used for regeneration and/or the activated carbon are heated.
An activated carbon filter of this type, whose activated carbon filling can be heated, is known from the German patent application 10 2009 020 703.1, the content of which is to be hereby incorporated by reference. In this case, electrically operated heating elements having a PTC-characteristic are used, so that in a convenient manner heating of the regeneration air stream or the activated carbon is ensured. For reasons of explosion-protection, the maximum temperature of the regeneration air stream and the activated carbon must be limited. To this end, a heating element having a PTC-characteristic is advantageously used which is heated as a result of an electric current flow and thus serves as a heating element. At the same time, the electric resistance of the heating element increases as the temperature increases, whereby the electric current flow and thus the heating output are reduced. This gives rise to a self-regulating procedure which leads to a controlled temperature of this heating element. The maximum temperature is set by the chemical build-up of the PTC-heating element. The interdependency of the current flow, heating output and temperature can be expressed in the following formulae:
I—electric current
U—electric voltage (is generally constant in the application)
R—electric resistance
P—electric output (corresponds to the heating output)
SinceI=U/R the current flow is reduced when voltage is constant, if the resistance increases as a result of an increase in temperature.P=U2/R If the resistance increases, the heating output is reduced when voltage is constant.P=I2×R 
It is apparent from this that when voltage is constant the heating output is proportional to the square of the current consumption of an electric element having a PTC-characteristic. It can be assumed that voltage will be constant in the application in a passenger vehicle, since it can be presumed that in this case a uniform vehicle electric system voltage is provided. However, since the influence of the variable voltage upon the current flow and the output is also expressed in the formulae above, a variable voltage can also be taken into consideration by means of a voltage measurement.
A particular characteristic of the electric devices having a PTC-characteristic (referred to hereinafter as PTC-elements) is that at the beginning of the heating phase a high electric current flow always flows through the elements and is reduced after a short time. This behavior is virtually independent of the PTC-temperature during the beginning of the heating phase and of whether the element has air to be heated passing through it.
An On-Board-Diagnosis (OBD) law which demands monitoring of exhaust gas-relevant electric components has been in force in California since 1988. In 1994 this law was tightened in California by OBD II. Hereafter, all components which are used for monitoring emission-relevant components are also to be monitored. Since, as described above, heating of the air used for regeneration and/or the activated carbon is provided for specific vehicles and the heating has a significant influence upon the fuel emissions of the vehicle, it is necessary to monitor that the heating operates correctly.